WO2008142136A2

WO2008142136A2 - Discriminating magnetic material properties

Info

Publication number: WO2008142136A2
Application number: PCT/EP2008/056316
Authority: WO
Inventors: Luc Van Bockstal; Jos Deschagt
Original assignee: Metis Instruments & Equipment Nv
Priority date: 2007-05-22
Filing date: 2008-05-22
Publication date: 2008-11-27
Also published as: WO2008142136A3; GB0709725D0

Abstract

A method and system are described for determining the contribution of induced currents in the magnetic moment of a sample. The method comprises performing a first measurement of a first magnetic moment of the sample using a first magnetic field pulse wherein the magnetization of the sample is reversed and wherein the first measured signal contains contributions from a non- reversing change in magnetization and an induced current. The method also comprises performing a second measurement of a magnetic moment of the sample wherein the magnetization of the sample is not reversed and wherein the second measured signal contains contributions from a reversible change in magnetization and the induced current.

Description

Discriminating magnetic material properties

Technical field of the invention

This invention relates to a technique and apparatus for measuring magnetic properties. It has uses in the testing of magnetic material, such as determining the hysteresis loop of a magnetic material.

Background of the invention

Permanent magnet materials are often used in devices to transform electrical energy into mechanical. In these devices, mechanical force is generated by the interaction of the magnetic field of a permanent magnet and the magnetic field generated by electrical current flowing through a coil. A typical example of such a device is a permanent magnet motor where current flowing through a winding generates a field which interacts with a field generated by a permanent magnet to drive a rotor. Another example is a loudspeaker where a winding is freely movable with respect to a permanent magnet. An amplified audio signal passes through the winding and the resulting magnetic field causes the winding and a loudspeaker cone to generate an audio signal. The permanent magnet is thus often used with a counteracting external field. For the stability of the device, it is desirable to know that the permanent magnet material used within the device can withstand the external magnetic field without deterioration of its magnetic state. With increasing strength of the counteracting magnetic field, the magnetization direction becomes unstable and the magnetization will ultimately reverse. The ability to withstand an external field is described in the technical literature with a set of curves of the magnetic moment per unit volume (the magnetization M) as a function of the magnetic field H. These curves are called hysteresis curves as they describe the memory of the magnetic state of a material and its resilience to change.

For typical applications, it is the behavior of the magnetization combined with a counteracting magnetic field which is of importance. On the graph of magnetization versus external field, this region of importance is located in the second quadrant. The parameter describing the magnetic field required to start reversing the magnetization of the material is the coercive field HcJ. Figure 1 shows the second quadrant of a typical hysteresis curve for a permanent magnetic material.

The advent of strong permanent magnetic materials such as NdFeB and SmCo created a revolution in devices because magnetic fields could be generated with significantly less material, thus saving weight and volume. Using such materials, it is now possible to produce a permanent magnet material in which the coercive field is larger than 2 MA/m.

In order to produce a hysteresis graph it is necessary to test a magnetic material by generating a magnetic field (i.e. a coercive field) over a wide range of strengths. With coercive fields above 2 MA/m, the generation of the magnetic field capable of measuring the full hysteresis curve requires special techniques. Conventional laboratory electromagnets with table-top sized iron yokes and pole shoes can achieve a field of 1.6 MA/m. Above this figure, generating the field can require in excess of 10 kW of electrical power. Superconducting magnets can achieve fields in the range 10 MA/m but they require special cryogenics. They are therefore relatively expensive in purchase and running costs. Magnetic fields generated from pulsed power supplies such as capacitor banks circumvent the need for large installed electrical power as the power is derived from an energy storage power supply. The consequence is that the pulse is limited in time by the available stored energy. At present pulsed field installations at research facilities reach fields up to 50 MA/m and above. Laboratory equipment based on compact capacitive discharges units reach fields up to 25 MA/m. In view of this, it is desirable to generate the magnetic fields required for measuring the complete hysteresis of highly coercive materials by using capacitive energy storage discharges.

Nowadays, a hysteresis meter based on a magnetic fields generated with a capacitive discharge is becoming accepted as a measurement tool for determining the hysteresis curve of permanent magnet material, as indicated in IEC TR 62331 "Pulsed field magnetometry", Technical Report of the International Electrotechnical Committee. Instruments capable of measuring hysteresis loops up to fields of 10 MA/m are becoming available. In these instruments, a sample of magnetic material is placed in a solenoid field coil. The instruments generate magnetic field pulses either with a half sine shape, measuring one half of the (symmetric) hysteresis curve, or with a full sine shape, measuring the full hysteresis curve. The magnetization of the sample is measured with pick-up coils. Due to the pulsed nature of the measurement technique, the change of the magnetic field will induce a current in the sample that counteracts the change in magnetic field. The magnetic moment associated with the induced current depends on the size of the sample, its conductivity and the rate of change of the B field .

Note that for magnetic material properties, H is magnetic field, B is the magnetic flux density and M is the volume magnetization. These properties relate via

B = μ_o ( H + M ) eq.1 where μ₀ is the magnetic permeability of vacuum.

The measurement of the magnetic moment of the sample in a pulsed system thus includes a contribution from the induced currents besides the properties of the magnetic material.

A typical pulsed field hysteresis meter would consist of a 20-40 kJ capacitive discharge supply with a solenoid with free bore around 50 mm in which fields up to 5 MA/m are generated. The pulse duration of the half sine pulse is of the order 10-20 ms. For a sample with a diameter of 30-40 mm, the moment from the induced currents is of the order 5 % of the moment of the magnetic material of a typical NeFeB permanent magnet. The induced currents are largely proportional to the rate of change of the B field and require only 10's of microseconds to recover from a transient.

One solution to separate the contribution of the induced currents from the magnetic properties of the material is to use capacitive discharge pulses with different rate of change, as described in US patent 5565774. Generating pulses with different rate of change is not trivial and increases the complexity of the measurement system. Summary of the invention

It is an object of embodiments of the present invention to provide good apparatus or methods for determining magnetic material properties. It is an advantage of embodiments according to the present invention that techniques and systems are provided for discriminating magnetic material properties. It is an advantage of embodiments of the present invention that techniques and apparatus are presented for determining the contribution from induced currents using magnetic field pulses. These magnetic field pulses may be generated with discharges creating the same rate of change of the B- field. A first aspect of the present invention provides a technique for determining the contribution of the induced currents in the magnetic moment of the sample, the technique comprising: at least a first measurement of the magnetic moment of the sample using a first magnetic field pulse wherein a magnetization of the sample is reversed and wherein the first measured signal contains contributions from a non-reversible change in magnetization and an induced current; and a second measurement of the magnetic moment wherein the magnetization of the sample is not reversed and wherein the second measured signal contains contributions from a reversible change in magnetization and an induced current. It is an advantage of embodiments according to the present invention that compensation of induced currents, also referred to as eddy currents, can be determined via direct measurement of the sample. It is an advantage that in this way, for example the geometry of the sample can be taken into account. It is an advantage of embodiments according to the present invention that no reference sample is required. It is an advantage of embodiments according to the present invention that the induced current signals can be determined substantially apparatus independent. It is an advantage of some embodiments according to the present invention that a pick-up coil system and numeric integrator therefore can be used. It is an advantage of embodiments according to the present invention that the induced currents are not measured independently but that these can be observed simultaneously with the magnetic hysteresis signal of the sample, therefore resulting in more accurate results. It is an advantage of embodiments according to the present invention that the induced currents can be determined and/or compensated taking into account the sample properties, such as for example the fact that the reversible part of a static hysteresis loop should be closed, the fact that the reversible part of a hysteresis loop is observed when the sample is fully saturated and/or the fact that the dynamic hysteresis loop, as measured in a pulsed-field magnetization meter, shows extremities that are not fully closed when the sample is fully saturated due to induced currents. It is an advantage of embodiments according to the present invention that the width of the loop at the extremeties when the sample is fully saturated can be used to determine the contribution of the eddy currents when the sample is not fully saturated.

The second measurement may be made using a part of the results (a second signal) obtained using the first magnetic pulse (e.g. where the magnetization direction of the sample is not reversed during both a rising and falling magnetic field pulse) or by using a second magnetic field pulse to generate the second signal. The first magnetic field pulse has a certain shape, i.e. a rising and falling shape and can have a symmetrical form such as a half sine shape. In any of the pulses used in the present invention, the magnetic field ramps up in intensity and then decays. One requirement for the applied field is that it is ramped up and down. In an embodiment of the present invention it can be a unipolar pulse. The second magnetic field pulse can be a similar rising and falling pulse, e.g. an identical magnetic field pulse to that of the first magnetic field pulse. In a section of the measured signals where the magnetization of the sample is not reversed (that is either sensed values generated by the first or second magnetic pulses), for every point in the up- going field ramp, there is a point in the down-going field ramp where the field is the same; the difference between the measured signals at these points is solely, or largely, due to the difference in the measured magnetic moment attributable to the induced currents because at these points, the signal from the (reversible value of) magnetization is the same. As this difference is proportional to the rate of change of the B field, the contribution from the induced current in the signal can be determined. Subtracting at given field strengths the thus determined contribution of the induced current from the first signal obtained in the first measurement yields a signal from the magnetic material of the sample without the induced current. Differences in the rate of change of the field can be accounted for by properly scaling the contributions with the actual rate of change of the B field.

In an embodiment a separate second measurement is made using a second pulse to investigate the section of the hysteresis curve where the magnetic moment of the sample is reversible in value, i.e. where changing the field strength up and down causes the magnetic moment to go up and down. In another embodiment, only one pulse is used and a separate part of the first measurement is used, i.e. a section of the hysteresis curve where the value of the magnetic moment of the sample is reversible, i.e. it goes up and down reversibly without change of magnetisation direction.

For samples which show irreversible demagnetization without applied field, the technique can be applied by carefully selecting the region where the difference in the signal in the second measurement varies monotonously with the rate of change of the B field and by extrapolating the monotonous behavior to the region where irreversible magnetization changes occur.

A second aspect of the invention provides a test apparatus for testing a sample of magnetic material, comprising means for generating a first magnetic field pulse wherein the magnetization of the sample is reversed, means for performing a first measurement of the magnetic moment using the first magnetic field pulse to generate a first signal, wherein the first measured signal contains contributions from an non-reversible change in magnetization and an induced current; and means for generating a second signal wherein the magnetization direction of the sample is not reversed and wherein the second measured signal contains contributions from a reversible change in magnetization and an induced current. The second signal can be derived from the first measurement using a section of the hysteresis curve where there is no reversing of the magnetization. This is for example a region around the saturation. In another embodiment, the means for generating a second signal can include means for generating a second magnetic field pulse wherein the magnetization of the sample is not reversed. The second signal is then derived from this second measurement The means for generating a first magnetic field pulse generates the magnetic field pulse with a certain shape, i.e. a rising and falling shape and this can have a symmetrical form such as a half sine shape. In the pulse, the magnetic field ramps up in intensity and then decays. If only one pulse is to be used, this pulse is preferably sufficiently large to send the sample into saturation. By this means a region of the hysteresis curve is measured in which the value of the magnetic moment can go up and down without change of magnetisation direction. In the other embodiment the means for generating a second magnetic field pulse generates the second magnetic field pulse with a similar rising and falling form, e.g. an identical magnetic field pulse to that of the first magnetic field pulse.

Further, the apparatus comprises a means for processing, e.g. arithmetic processing of the first and/or second signals, e.g. subtracting the second signal from the first signal, whereby the signals may be adjusted or normalized before such a subtraction. For every point in the up-going field ramp, there is a point in the down-going field ramp where the applied field is the same; the difference between the measured signals at these points is solely, or largely, due to the difference in the moment deriving from the induced currents because at these points, the signal from the (reversible) magnetization is the same. As this difference is proportional to the rate of change of the B field, the contribution from the induced current in the signal can be determined.

The means for arithmetic processing can be adapted to subtract at given field strengths the contribution of the induced current as determined from the second measurement from the signal obtained in the first measurement to thereby yield a value related to, or a signal derived from the magnetic material of the sample without the induced current. The means for arithmetic processing can be adapted to allow for differences in the rate of change of the field by means for scaling the contributions with the actual rate of change of the B field. To achieve this a means for measuring the field B may be provided which provides its output to the means for arithmetic processing.

For samples which show irreversible demagnetization without applied field, the apparatus can be adapted to select the region where the difference in the signal in the second measurement varies monotonously with the rate of change of the B field and to extrapolate the monotonous behavior to the region where irreversible magnetization changes occur.

The test apparatus may comprise at least one generator for generating a varying magnetic field within a test zone, e.g. a coil or coils and an energy source and at least one field sensor, e.g. first and second pick-up coils positioned within the test zone.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings.

Brief description of the drawings Fig 1 : shows an example of a hysteresis curve for a permanent magnetic material

Fig 2: shows an example of an overall test apparatus for measuring the magnetic moment of a sample in the presence of a magnetic field in accordance with an embodiment of the invention Fig 3: shows all quadrants of the hysteresis loop as measured in accordance with an embodiment of the invention. Line 1 is the original data set of the irreversible magnetization change versus applied field, obtained by applying a magnetic field in the opposite direction of the magnetization; line 2 is the data set corresponding to the reversible magnetization change, obtained by applying a magnetic field in the direction of the magnetization; the vertical width of the loop corresponds to the contribution of the eddy currents. Line 3 represents the hysteresis of the length of material without induced currents. Fig. 4 illustrates a computing system as can be used for implementing a method according to an embodiment of the present invention.

Description of illustrative embodiments

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

It should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

Furthermore, some of the embodiments are described herein as a method or combination of elements of a method that can be implemented by a processor of a computer system or by other means of carrying out the function. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, an element described herein of an apparatus embodiment is an example of a means for carrying out the function performed by the element for the purpose of carrying out the invention.

In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

The invention will now be described by a detailed description of several embodiments of the invention. It is clear that other embodiments of the invention can be configured according to the knowledge of persons skilled in the art without departing from the true spirit or technical teaching of the invention, the invention being limited only by the terms of the appended claims.

Figure 2 shows an embodiment of a test apparatus for measuring the magnetic moment of a sample of magnetic material within a background magnetic field. Figure 2 shows the overall apparatus. The system comprises a magnetic field generating means or magnetic field generator for generating a magnetic field. The magnetic field generator may be adapted for generating a ramped magnetic field. The magnetic field generator may for example comprise a large coil 4, such as a solenoid, that is connected to a power supply 10 which is capable of injecting sufficient electrical current into the generator to generate a ramped magnetic field at coil 4. An up-ramp and down-ramp of magnetic field advantageously is used. More than one generator may be used, e.g. more than one coil. A controller 16 may be provided to control the operation of the magnetic field generator. The magnetic field will be referred to as the applied field, the background field or the coercive field, as appropriate. A pulse energy supply 10 is provided by an energy storage supply is, e.g. a capacitive discharge unit with a stored energy of 5-50 kiloJoule operating at a maximum voltage of 1 -10 kV and delivering peak currents of 1-10 kA. The coil 4 may for example be a solenoid wound such as for example a copper wire. It may have an inductance, e.g. of 1 mH. This power supply is preferably capable of generating a magnetic field of more than 1 MA/m with ramping up and a ramping down field strength. The pulse may be a unipolar pulse such as a half- sine pulse with duration, e.g. of 10-50 milliseconds. For the present invention, the one requirement for the field is that it is ramped up and down.

In use, the area within coil 4 is exposed to the pulsed magnetic field. Positioned within the coil 4 is a support such as a hollow cylindrical support 5. Support 5 has a diameter which is sufficient to accommodate a test sample 1. For example, this will be a length of permanent magnetic material. Two sensors 2, 3 are mounted on the support 5 to generate signals representative of the magnetic moment of the sample and the strength of the applied field, respectively. These output signals are fed to a data acquisition unit 14, e.g. containing a processor able to carry out mathematical manipulations on these signals. Typically the sensors 2, 3 will be inductive pick-up loops. A first sensor 2 will detect primarily the flux from the magnetization of the sample; a second sensor 3 will detect primarily the flux from the applied magnetic field. More sensors may be used, e.g. more coils whereby the signal from the coils may be combined in ways to eliminate the effects of stray fields and other types of error as is known to the skilled person. The present invention includes within its scope that the sensors measure a signal containing a mixture of both signals and a signal or data processor in the data acquisition unit 14 (or elsewhere) is provided for signal or data manipulation to separate both of the signals. In the following methods according to the present invention are described but the present invention also includes the means for carrying out one, some or all of the individual steps of these methods, e.g. using the equipment described schematically for Figure 2.

In a first embodiment, 2 data sets are derived from one measurement, i.e. one magnetic pulse as well as data manipulation on the data sets using the data acquisition unit 14. The first data set is obtained by applying a magnetic field on a sample of magnetic material wherein the initial direction of the magnetization is opposite to the applied field. We will refer to this data set as the first measurement data set. During the up-slope of the field, the magnetization of the material is reversed. During the down-slope of the field, the magnetization will remain in the same direction. The behavior of the magnetization, i.e. value of observed magnetic moment during the down slope is reversible, in general, i.e. goes up and down with the applied field without change of magnetisation direction. The second data set is obtained by applying a magnetic field on the sample wherein the initial direction of the magnetization is in the direction of the applied field. We will refer to this data set as the second measurement data set.

In general, the behavior of the magnetization is reversible in a portion of the first measurement carried out near the saturation point (region "X" in Fig. 3). The magnetization during the up-slope and down-slope in this region is then equal or nearly equal at identical fields. The moment of the sample, which contains both sample magnetization and induced currents, will be mainly reversible but will show a difference or a value that does not exactly coincide on the up ramp and down ramp (described in this document as an irreversible behavior) which is solely, or largely, due to induced current. Due to the monotonously changing behavior of the induced currents, the second measurement will look like a loop in the upper right (first) quadrant (see Line 2 in Figure 3, region "X"). In the following methods according to the present invention are described but the present invention also includes the means for carrying out individual steps of these methods, e.g. using the equipment described schematically for Figure 2. In a second embodiment 2 data sets are obtained from 2 measurements, i.e. using two magnetic pulses, as well as data manipulation on the data sets using the data acquisition unit 14. The first data set is obtained by applying a magnetic field on a sample of magnetic material wherein the initial direction of the magnetization is opposite to the applied field. We will refer to this data set as the first measurement data set. During the up- slope of the field, the magnetization of the material is reversed. During the down-slope of the field, the magnetization will remain in the same direction. The behavior of the magnetization during the down slope is reversible, in general, i.e. the magnetic moment changes up and down with the applied field. The second data set is obtained by applying a second magnetic field pulse on the sample wherein the initial direction of the magnetization is in the direction of the applied field. We will refer to this data set as the second measurement data set.

In general, the behavior of the magnetization is reversible in the second measurement, i.e. the magnetic moment changes up and down with the applied field. The magnetization during the up-slope and down-slope is then equal or nearly equal at identical fields. The magnetic moment of the sample, which contains both sample magnetization and induced currents, will be mainly reversible (i.e. the magnetic moment changes up and down with the applied field) but will show a difference or will not have the same value at equal field strengths (described as an irreversible behavior in this document) which is solely, or largely, due to an induced current. Due to the monotonous behavior of the induced currents, the second measurement will look like a loop in the upper right (first) quadrant (see Line 2 in Figure 3).

The induced currents are determined by the geometry of the sample, the conductivity and the rate of change of the B field. For a monotonously rising or falling field sweep, the induced currents are proportional to the rate of change, (except at the start of the pulse when there is a redistribution of the eddy currents related to the inrush of the field in sample). The monotonously changing behavior results in a total magnetic moment m for a given field H m (H) = m_m(H) + m,(H) eq.2 where m_m is the contribution of the magnetic material and m, is the contribution from the induced currents. In the monotonously changing regime, the contribution of the induced currents is proportional to the rate of change of the flux density B m (H) = m_m(H) + x(H) dB/dt(H) eq.3

For identical H values the present invention includes methods and apparatus for the measurement of the magnetic moment at the rising field and at the falling field. In particular the present invention provides methods and apparatus for determining the contribution of the induced currents in signals representing the magnetic moment of the sample by making a first measurement of the magnetic moment using a first magnetic field pulse wherein the magnetization of the sample is reversed and wherein a first measured signal contains the contributions from an irreversible change in magnetization and an induced current, and a second measurement of the magnetic moment wherein the magnetization of the sample is not reversed and wherein a second measured signal contains the reversible change in magnetization and the induced current. The second signal can be determined from the first measurement or from a separate application of a second magnetic field pulse. In case of the second measurement which starts with a magnetic moment already aligned in the direction of the field, the moment m⁺ of the rising field and the moment m^" of the falling field will be determined by the same magnetization m_m(H) and different induced currents. The difference between these 2 signals in the second measurement is then: m⁺ (H) - m^" (H) = + x(H) ( dB⁺/dt(H) - dB7dt(H) ) eq.4

This allows determination of the proportionality factor x (H) and determination of the contribution of the induced currents: m,(H) = ( m⁺ (H) - m^" (H) ) * dB⁺/dt(H) / ( dB⁺/dt(H) - dBVdt(H) ) eq.5

Subtracting the contribution of the induced currents determined from the second measurement from the total magnetic moment m as measured by the first measurement, yields the contribution of the magnetic material mm (H) = m(H) - m,(H) eq.6 which is the hysteresis curve of the magnetic material versus magnetic field H. The present invention includes apparatus for carrying out these operations. Samples showing irreversible demagnetization

For samples which show irreversible demagnetization without applied field, a method and apparatus according to an embodiment of the present invention includes selecting a region where the difference in the second signal in the second measurement varies monotonously with the rate of change of the B field and by extrapolating this monotonous behavior to the region where irreversible magnetization changes occur. The region can be easily identified because the induced currents are largely proportional to the rate of change of the B field. Any significant deviation, typically of the order 10-30 %, of this monotonous behavior is due to irreversible demagnetization in the magnetic moment of the material.

When analyzing the data set of the second measurement using the equation eq.6, the proportionality factor x (H) shows a monotonous, nearly constant, behavior at high fields. For deviation at lower fields, the monotonous behavior can be extrapolated using classical extrapolation techniques, (constant, linear...).

By using the extrapolation, it is possible to perform a correction for the induced currents over the entire data set of the first measurement. Transients in the induced currents at the beginning and the end of the pulse.

For a monotonously changing applied field, the induced currents are proportional to the rate of change of the B field. Any change in the rate of change gives rise to transient which is equilibrated quickly. For a sample of 30 mm diameter the transient lasts around 20-50 μs after which the induced currents are in a steady state. For pulsed fields with a half-sine shape, these transients are observed at the beginning and the end of the pulse. The transients related to the start and end of the pulse are easily identified and represent in general a small fraction of the total data set. They can be dealt with either by a smoothing filter to suppress their influence or by identifying the transient shape - which has an exponential shape in first order - in the first and second measurement and subtracting these shapes only in the region where this transient occurs.

The above-described method embodiments of the present invention may be completely or partly implemented in a processing system 100 such as shown in Fig. 4. Fig. 4 shows one configuration of processing system 100 that includes at least one programmable processor 103 coupled to a memory subsystem 105 that includes at least one form of memory, e.g., RAM, ROM, and so forth. It is to be noted that the processor 103 or processors may be a general purpose, or a special purpose processor, and may be for inclusion in a device, e.g., a chip that has other components that perform other functions. Thus, one or more aspects of the present invention can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. The processing system may include a storage subsystem 107 that has at least one disk drive and/or CD-ROM drive and/or DVD drive. In some implementations, a display system, a keyboard, and a pointing device may be included as part of a user interface subsystem 109 to provide for a user to manually input information. Ports for inputting and outputting data also may be included. More elements such as network connections, interfaces to various devices, and so forth, may be included, but are not illustrated in Fig. 4. The various elements of the processing system 100 may be coupled in various ways, including via a bus subsystem 113 shown in Fig. 4 for simplicity as a single bus, but will be understood to those in the art to include a system of at least one bus. The memory of the memory subsystem 105 may at some time hold part or all (in either case shown as 111 ) of a set of instructions that when executed on the processing system 100 implement the steps of the method embodiments described herein. Thus, while a processing system 100 such as shown in Fig. 4 is prior art, a system that includes the instructions to implement aspects of the methods for determining the contribution of induced currents in the magnetic moment of a sample is not prior art, and therefore Fig. 4 is not labelled as prior art.

The present invention also includes a computer program product which provides the functionality of any of the methods according to embodiments of the present invention when executed on a computing device. Such computer program product can be tangibly embodied in a carrier medium carrying machine-readable code for execution by a programmable processor. The present invention thus relates to a carrier medium carrying a computer program product that, when executed on computing means, provides instructions for executing any of the methods as described above. The term "carrier medium" refers to any medium that participates in providing instructions to a processor for execution. Such a medium may take many forms, including but not limited to, non-volatile media, and transmission media. Non volatile media includes, for example, optical or magnetic disks, such as a storage device which is part of mass storage. Common forms of computer readable media include, a CD-ROM, a DVD, a flexible disk or floppy disk, a tape, a memory chip or cartridge or any other medium from which a computer can read. Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to a processor for execution. The computer program product can also be transmitted via a carrier wave in a network, such as a LAN, a WAN or the Internet. Transmission media can take the form of acoustic or light waves, such as those generated during radio wave and infrared data communications. Transmission media include coaxial cables, copper wire and fibre optics, including the wires that comprise a bus within a computer.

It is to be understood that although preferred embodiments, specific constructions and configurations, as well as materials, have been discussed herein for devices according to the present invention, various changes or modifications in form and detail may be made without departing from the scope of this invention as defined by the appended claims. For example, any formulas given above are merely representative of procedures that may be used. Functionality may be added or deleted from the block diagrams and operations may be interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention.

Claims

1.- A method for determining the contribution of induced currents in the magnetic moment of a sample, comprising: a first measurement of a first magnetic moment of the sample using a first magnetic field pulse wherein the magnetization of the sample is reversed and wherein the first measured signal contains contributions from a non- reversing change in magnetization and an induced current; a second measurement of a magnetic moment of the sample wherein the magnetization of the sample is not reversed and wherein the second measured signal contains contributions from a reversible change in magnetization and the induced current.

2.- The method of claim 1 , wherein the second measurement is performed using a second magnetic field pulse.

3.- The method of claim 1 or 2, wherein the first magnetic field pulse or the second magnetic field pulse have rising and falling intensity values.

4.- The method of claim 3, wherein the rising and falling intensity values have a symmetrical form such as a half sine shape.

5.- The method of claim 3 or 4 wherein the rising and falling intensity values are unipolar.

6.- The method of any of claims 3 to 5, wherein for a point in an up-going field ramp there is a point in a down-going field ramp where the field is equal, further comprising measuring a difference between the measured second signals at these points.

7.- The method of claim 6, further comprising determining the contribution from the induced current present in the first signal.

8.- The method of claim 7, comprising: subtracting at given field strengths the contribution of the induced current as determined from the second measurement from the first signal obtained in the first measurement to thereby yield a signal representative of the magnetic material of the sample without the induced current.

9.- The method of any of the claims 6 to 8, wherein differences in a rate of change of the field is accounted for by scaling the contributions with an actual rate of change of a magnetic flux density (B).

10.- The method according to any previous claim, wherein the sample shows irreversible demagnetization without applied field, further comprising selecting a region where a difference in the second signal in the second measurement varies monotonously with a rate of change of a magnetic flux density (B) and extrapolating the monotonous behavior to a region where irreversible magnetization changes occur.

11.- The method according to any previous claim wherein measuring the first signal of magnetic moment versus applied field is measured with a field coil driven by a pulsed generator generating a first unipolar pulse, wherein the magnetization of the sample is reversed during the pulse.

12.- The method of claim 11 wherein measuring the second signal of magnetic moment versus applied field is measured with a field coil driven by a pulsed generator generating the first unipolar pulse in a region where the magnetization of the sample is not reversed during application of the first pulse.

13.- The method of claim 11 wherein measuring the second signal of magnetic moment versus applied field is measured with a field coil driven by a pulsed generator generating a second unipolar pulse, the magnetization of the sample is not reversed during the second pulse.

14.- The method of claim 12 or 13, further comprising determining the difference in the second signal at identical fields during up and down ramping of the first or second unipolar field.

15.- The method of claim 14, wherein the signal difference is determined at a given field split proportionally to the field sweep rate over the magnetic field at the up ramp and the magnetic field at the down ramp of the unipolar pulse, i.e. corresponding to the induced currents.

16.- The method of any of the claims 14 or 15, wherein the signal difference is subtracted from the first signal, yielding a signal representative of the magnetic hysteresis from which the signal of the induced currents is subtracted.

17.- The method of any of claims 14 to 16 wherein the signal difference at a given field is extrapolated to the low field region for the case that a magnetic material sample does exhibit irreversible behavior in the magnetic material properties.

18.- The method of claim 17, wherein the signal difference is analyzed with respect to the monotonous behavior in order to detect non-reversible behavior in the magnetic material properties.

19.- An apparatus for determining the contribution of induced currents in the magnetic moment of a sample, comprising: a first magnetic field generator for generating a first magnetic pulse, means for a first measuring of a magnetic moment of the sample using the first magnetic field pulse wherein the magnetization of the sample is reversed and wherein the first measured signal contains contributions from a non-reversible change in magnetization and an induced current; second means for measuring a second signal wherein the magnetization of the sample is not reversed and wherein the second measured signal contains contributions from reversible changes in magnetization and an induced current.

20.- The apparatus of claim 19, further comprising a second magnetic field generator for generating a second magnetic pulse, the second means including means for measuring a magnetic moment of the sample using the second magnetic field pulse.

21.- The apparatus of claim 20, wherein the first magnetic field generator is the same as the second magnetic field generator, and the first magnetic field generator is adapted to generate the first and second magnetic pulses in time sequential manner.

22.- The apparatus of claim 20 or 21 , wherein the first and/or second generator generate the first magnetic field pulse and the second magnetic field pulse, respectively having rising and falling intensity values.

23.- The apparatus of claim 22, wherein the first and/or second generator generate the first magnetic field pulse and the second magnetic field pulse, respectively having the rising and falling intensity values in a symmetrical form such as a half sine shape.

24.- The apparatus of any of the claims 20 to 23, wherein the first and/or second generator generate a first unipolar magnetic field pulse and a second unipolar magnetic field pulse, respectively.

25.- The apparatus according to any of the claims 20 to 22, wherein for a point in an up-going field ramp of the first or second pulse there is a point in the down-going field ramp where the field is equal; further comprising means for measuring the difference between the measured first or second signals at these points.

26.- The apparatus of claim 25, further comprising means for determining the contribution from the induced current in the first signal.

27.- The apparatus of claim 26, comprising: means for subtracting at given field strengths the contribution of the induced current as determined from the second measurement, from the first signal obtained in the first measurement to thereby yield a signal representative of the magnetic material of the sample without the induced current.

28.- The apparatus of claim 26 or 27, comprising means for accounting for differences in the rate of change of the field by scaling the contributions with the actual rate of change of the magnetic flux density (B).

29.- The apparatus according to claim 28, further comprising means for measuring the rate of change of the magnetic flux density (B).

30.- The apparatus according to any of the claims 20 to 29, wherein the sample shows irreversible demagnetization without applied field, further comprising means for selecting a region where the difference in the second signal in the second measurement varies monotonously with the rate of change of the magnetic flux density (B) and extrapolating the monotonous behavior to a region where irreversible magnetization changes occur.

31.- The apparatus according to any of the claims 20 to 30 wherein the first generator is a field coil driven by a pulse generator generating a first unipolar pulse, wherein the magnetization of the sample is reversed during the pulse.

32.- The apparatus of claim 31 wherein the second generator is a field coil driven by a pulse generator generating a second unipolar pulse, the magnetization of the sample is not reversed during the second pulse.

33.- The apparatus of claim 31 or 32, further comprising means for determining the difference in the second signal at identical fields during up and down ramping of the unipolar field.

34.- The apparatus of any of the claims 31 to 33, further comprising means for determining the signal difference a given field split proportionally to the field sweep rate over the magnetic field at the up ramp and the magnetic field at the down ramp of the unipolar pulse, i.e. corresponding to the induced currents.

35.- The apparatus of any of the claims 27 to 34, wherein the means for determining the signal difference is adapted to subtract the signal difference from the first signal, yielding a signal representative of the magnetic hysteresis from which the signal of the induced currents has been subtracted.

36.- The apparatus according to any of the claims 27 or 35, wherein the means for determining the signal difference is adapted to determine the signal difference at a given field and to extrapolate to a low field region for the case that a magnetic material sample does exhibit irreversible behavior in the magnetic material properties.

37.- The apparatus of claim 36, further comprising means for analyzing the signal difference with respect to the monotonously varying behavior in order to detect irreversible behavior in the magnetic material properties of the sample.